One conventional way of high density packaging of electronic components utilizes a printed circuit board having holes in it which allow the leads of the electronic components to protrude through the board. The components are situated on one side of the board with their leads extending through the holes in the board to the opposite side. The leads are usually soldered to the printed wiring by the wave soldering process in which the crest of a wave of molten solder progresses along the bottom of the board and wets with molten solder the leads and the printed wire terminals that are to be connected. The solder, upon solidifying, provides a good electrical path lead and the terminal. Because only one side of the printed circuit board is used for virtually all of the soldered connections and because wave soldering is so prevalent, it is accepted practice to mount all the electronic components on the other side of the board. To facilitate that manner of packaging electronic components, most electric components having many leads, such as integrated circuit devices, are produced as DIP's--that is, are produced with two parallel rows of leads which are long enough to enable the leads to extend through the holes of the circuit board. The term DIP is an acronym for dual inline package.
The number of electronic components that can be mounted on a printed circuit board or on a substrate having electrically conductive pads to which the leads are to be attached, can be increased by mounting electronic components on both sides of the board or substrate. However, by placing electronic components on both sides of the substrate or board, the wave soldering process is interferred with to such an extent that in most instances it cannot be successfully used to make the requisite connections.
There is currently an increasing tendency to package multi-lead electronic components by the method of surface mounting. In that method, electronic devices known as SMP's are used rather than DIP's. The term SMP is an acronym for surface mounted package. The SMP device has protruding leads for attachment to pads on the substrate or board which are located on the same side as the side on which SMP device is mounted. Because the leads of the SMP device do not extend through holes in the substrate or board, those leads need be only long enough to reach the pads. Consequently, the leads of SMP device are usually short stubby wires that protrude from the SMP device. The surface mounting technique allows components to be mounted on both sides of the board or substrate. For ease of exposition, the terms "board" and "substrate" are hereinafter used interchangably to mean any kind of support on which electronic components can be mounted and which has electrically conductive paths providing terminals or pads to which the leads of the mounted components can be secured to provide electrical connections.
In one widely used method of surface mounting, the SMP devices are attached by an adhesive (such as an epoxy glue) to the board. When all (or almost all) the components have been attached to the board, the entire "populated" board is placed in an oven whose temperature is high enough to cause pre-positioned solder to reflow locally around each joint. That method, while it enables all the soldered connections to be made in a single heating step, has a number of significant drawbacks, among which are:
(a) most of the energy in heating the entire board to the solder reflow temperature is wasted because the only locations that need be heated are in the vicinity of the joints to be soldered; PA1 (b) in the cool-down phase, stresses are introduced in the solder joints because of the difference in the coefficients of thermal expansion between the board and the SMP devices; and PA1 (c) the exposure of the SMP devices to the high temperature in the oven may damage the integrated circuits within those devices or reduce the life or reliability of those devices.
The last cited of those drawbacks is the most significant one because electronic devices in which the semiconductor is silicon or germanium or other heat sensitive material have critical heat limitations which must not be exceeded to preserve the reliability of the device and to avoid catastrophic failure.
To avoid the disadvantages of the surface mounting procedure in which the soldered connections are made by heating the populated board in an oven, it has been proposed to solder each joint, one or two at a time, by using a laser beam to heat the joint. That method of using a laser beam for "sequential" soldering is described in U.S. Pat. No. 4,327,277, granted on Apr. 27, 1982 to Kevin Daly.
The "sequential" soldering method is slow because a solder joint requires about 0.1 to 0.2 seconds to complete. Although the time can be shortened somewhat by using high power beams from the laser source, that time cannot be significantly shortened because the radiant energy deposited by the beam on the surface is turned into heat and that heat must have sufficient time to be conducted through the pre-positioned solder at the joint to insure the making of a reliable soldered connection. A typical SMP device has 24 to 48 leads so that the total time to make soldered connections by the "sequential" method is much too slow for high production in surface mounted packaging.
To avoid the tedious, time consuming method of bonding multiple leads one at a time, U.S. Pat. No. 3,632,955 to Cruickshank et al proposes to shape a beam of radiant energy into a pattern suitable for simultaneously bonding all the leads in one operation. To accomplish that purpose, a composite cylindrical lens is employed having a plurality of lens segments. A beam of radiant energy is directed onto the lens and is focussed into two pairs of parallel lines which form a rectangle that is congruent with the geometric arrangement of the leads to be bonded. The rectangle of radiant energy is directed onto the leads without having any of that energy incident upon the device from which the leads extend.
The Cruickshank et al bonding method is inflexible in that each different geometrical configuration requires its own composite cylindrical lens and because there is no provision for adjustment of the radiation pattern for individual leads. Further, in the Cruickshank et al bonding method, much of the radiant energy is wasted either by falling on areas between the leads or by absorption or reflection from a mask that intercepts that energy before it can reach the areas between the leads.